Free Astronomy Magazine September-October 2022

27 SEPTEMBER-OCTOBER 2022 ASTRO PUBLISHING This causes their emissions to be redshifted (shifted towards longer wavelengths). Interestingly, we can estimate how fast a galaxy is moving and, in turn, when it was formed based on how redshifted the emis- sion appears. ALMA is particularly well-suited for observing such red- shifts in galaxy emission. The international research team has observed the redshifted emissions in the distant galaxy MACS1149-JD1, or JD1 for short, which has led them to some interesting conclusions. “Be- yond finding high-redshift, namely very distant, galaxies, studying their internal motion of gas and stars pro- vides motivation for understanding the process of galaxy formation in the earliest possible universe,” ex- plained Richard S. Ellis, a professor at University College London. Galaxy formation begins with the accumulation of gas and proceeds with forming stars from that gas. With time, star formation progresses from the center outward, a galactic disk develops, and the galaxy ac- quires a particular shape. As star for- mation continues, newer stars form in the rotating disk while older stars remain in the central part. It is pos- sible to determine the evolutionary stage of the galaxy by studying the age of the stellar objects and the motion of the stars and gas. The team successfully measured small differences in the “redshift” from position to position inside the galaxy, showing that JD1 satisfied the criterion for a galaxy dominated by rotation. As already mentioned, the calculated rotational speed was about 50 kilometers per second, compared to the rotational speed of the Milky Way disk of 220 kilometers per second. The team also measured the diameter of JD1 at only 3,000 light-years, which is much smaller than that of the Milky Way at 100,000 light-years across. The galaxy the team observed is by far the most distant source yet found that has a rotating disk. To- gether with similar measurements of nearer systems in the research lit- erature, this has allowed the team to delineate the gradual develop- ment of rotating galaxies over more than 95% of our cosmic history. Furthermore, the mass estimated from the rotational speed was in line with the stellar mass estimated previously from the spectral signa- ture, and came predominantly from that of “mature” stars that formed about 300 million years ago. “This shows that the stellar population in JD1 formed at an even earlier epoch of the cosmic age,” said Takuya Hashimoto, an assistant professor at Univesity of Tsukuba. “The rotational speed of JD1 is much slower than those found in galaxies in later epochs and the Milky Way, and JD1 is likely at an initial stage of developing a rotational motion,” said Akio Inoue, a professor at Waseda University. With the James Webb Space Telescope, the team now plans to identify the locations of young and older stars in the galaxy to refine their scenario for its formation. C onceptual image of MACS1149- JD1 forming and spinning up to speed in the early universe. [ALMA (ESO/NAOJ/NRAO)] !